Method for forming thin film with a gas cluster ion beam

ABSTRACT

A method of forming a thin film on the surface of a substrate such as silicon, in which a gas cluster (which is a massive atomic or molecular group of a reactive substance taking the gaseous form at room temperature under atmospheric pressure) is formed and then ionized, and the cluster ions are then irradiated onto a substrate surface under an acceleration voltage to cause a reaction.  
     It is possible to form a high quality ultra-thin film having a very smooth interface, without causing any damage to the substrate, even at room temperature.

FIELD OF THE INVENTION

[0001] The present invention relates to a method for forming a thin film on the surface of a substrate with a gas cluster ion beam. More particularly, the present invention relates to a method for forming a thin film by gas cluster ion beam which is a group of reactive substance which is gaseous at the room temperature under the atmospheric pressure. The method is useful for the manufacture of a semiconductor or other electronic devices and for surface reforming of a functional material.

PRIOR ART AND PROBLEMS

[0002] A method for forming a thin film by irradiating monatomic or monomolecular ions onto a substrate surface has conventionally been used in practice. This method utilizes a high input energy of several keV because low-energy ion irradiation cannot give an adequate beam because of the space charge effect between ions.

[0003] In this conventional method, however, the use of ions having a large input energy makes it difficult to avoid damage to the substrate surface, and thus deterioration of semiconductor devices has been a major problem.

[0004] As a method for forming a thin film by causing reaction between a reactive substance, which is gaseous at the room temperature, and a semiconductor substrate, a thermal reaction method has been developed for forming an oxide or nitride film by heating the substrate to a high temperature in an atmosphere of the reactive substance. The thin film formed by this method has excellent interface and insulation properties, and the method has been industrialized for forming an insulating film or a capacitor insulating film for silicon semiconductor devices.

[0005] This method requires a low temperature to reduce the diffusion of impurities, which is desirable for integrated circuit devices, the chemical vapor deposition (CVD) method is adopted for this purpose.

[0006] In this case, however, it is necessary to heat the substrate surface to a temperature of at least 400° C., but this results in defects such as a low density of the resultant film as compared with that produced by the thermal reaction method and the presence of many unsaturated bonds on the interface between the substrate and the thin film.

[0007] The plasma CVD method is known as a method for forming a thin film at a low temperature. This method however involves such defects as a large amount of mixed impurities, occurrence of damages to the substrate surface by ions, and difficulty in controlling the film thickness of an ultra-thin film. Furthermore, it is not applicable to a transistor gate insulating film or a capacitor insulating film requiring high quality.

[0008] In the conventional techniques, as described above, the quality of the resultant thin film deteriorates as the process temperature becomes lower, and it is difficult to obtain an ultra-thin film useful for a hyperfine semiconductor circuit device such as ULSI.

[0009] There has consequently been a strong demand for a new method which permits formation of a high-quality thin film at a lower temperature, particularly at room temperature, without the need to heat the substrate and without damaging the substrate surface, as a fundamental technology for use in advanced electronics such as ULSI.

[0010] The present invention was developed in view of the circumstances as described above, and has an object to provide a method for forming a high-quality ultra-thin film having a smooth interface with the substrate free from a damage at the room temperature, which solves the drawbacks in the conventional methods for forming a thin film.

SUMMARY OF THE INVENTION

[0011] The present invention provides a method for forming a thin film on the surface of substrate with a gas cluster ion beam, which comprises the step of irradiating the surface of a substrate with ions of a gas cluster (which is a massive group of atoms or molecules of a reactive substance taking the gaseous form at room temperature under the atmospheric pressure) to cause a reaction with a substance of the substrate surface, thereby forming a thin film on the substrate surface.

[0012] More specifically, the invention comprises the steps of using a reactive substance, selected from the group consisting of oxides, nitrides, carbides, mixtures thereof, and mixtures thereof with an inert gas and causing a reaction between a gas cluster ion beam of this substance and a substance of the substrate surface, thereby forming a thin film on the substrate surface.

[0013] The invention provides also a method for forming a thin film based on a gas cluster ion beam, which comprises the steps of irradiating the surface of the substrate with the gas cluster ion beam to form a thin film and at the same time to planarize the surface.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014]FIG. 1 shows transmission type electron microscopic photomicrographs of a section of a silicon substrate by irradiating CO₂ monomer and CO₂ cluster ions onto the substrate surface thereof;

[0015]FIG. 2 shows an infrared absorption spectrum of a thin film formed by irradiating CO₂ monomer ions and CO₂ cluster ions onto a silicon substrate surface;

[0016]FIG. 3 shows a chart illustrating the relationship between the dose rate of CO₂ ions to be irradiated and the concentration of impurities in the substrate surface;

[0017]FIG. 4 shows a diagram illustrating the generation of clusters of various gases by means of a nozzle having cooling means;

[0018]FIG. 5 shows a graph illustrating the relationship between the supply pressure and the cluster beam intensity at various nozzle intensities for oxygen gas;

[0019]FIG. 6 shows a graph illustrating the relationship between the supply pressure and the cluster beam intensity at various nozzle intensities for nitrogen gas;

[0020]FIG. 7 shows a graph illustrating the relationship between the supply pressure and the cluster beam intensity at various nozzle intensities for argon gas for reference; and

[0021]FIG. 8 shows an infrared absorption spectrum of an oxide film formed by irradiating oxygen (O₂) gas cluster onto a silicon substrate surface.

DETAILED DESCRIPTION OF THE INVENTION

[0022] The method of the invention comprises generating a gas cluster (which is a massive group of atoms or molecules of a reactive substance which is a gas at room temperature under the atmospheric pressure), irradiating electrons onto this gas cluster to ionize same, and irradiating the thus generated gas cluster ions onto a substrate surface by selecting a beam of a particular size as required.

[0023] Irradiation is accomplished by accelerating the gas cluster ions under an acceleration voltage. Because a cluster usually consists of a group of several hundred atoms or molecules, each atom or molecule is irradiated as an extra-low temperature ion beam with up to several tens of eV and even under an acceleration voltage of 10 KeV. It is therefore possible to form a high-quality ultra-thin film with a very low degree of damage at room temperature. A thin film with the slightest content of impurities can be produced as the impurities are removed from the substrate surface by the effect of the gas cluster ions.

[0024] Another effective manner of irradiation is to appropriately select the number of component molecules in response to the substrate, the kind of substance of the substrate surface and the desired thin film composition.

[0025] The cluster itself can be generated by ejecting a pressurized gas through an expansion type nozzle into a vacuum unit, as already proposed by the present inventors. The thus generated cluster can be ionized by irradiating electrons.

[0026] In the present invention, the gas cluster ions are irradiated onto a substrate surface to planarize the surface and at the same time, the irradiated ions are caused to react with the substance of the substrate surface, thereby permitting formation of a thin film on the substrate surface.

[0027] The gaseous reactive substances taking a gaseous form at room temperature under the atmospheric pressure include, for example, oxygen, oxides such as CO₂, CO, N₂O, NO_(x), and C_(x)H_(y)O_(z), carbides, nitrides such as N₂ and NH_(x), sulfides, halides, hydrides such as AsH_(x) and SiH_(x), organometallic compounds such as metal carbonyls.

[0028] In the present invention, for example, the reactive substance is preferably oxygen or a carbon oxide, or a mixture thereof, or a mixture thereof with an inert gas substance, and the thin film formed can be an oxide film.

[0029] In the case of any of the gases including oxygen (O₂) and nitrogen (N₂), the present invention specifically proposes a method for generating gas cluster ions, which comprises the step of generating a gas cluster by means of a nozzle cooled by a coolant, and ionizing the resultant cluster.

[0030] The invention will be described in further detail by the following of examples.

EXAMPLE 1

[0031]FIG. 1 is transmission type electron microscopic photomicrographs of a silicon substrate section when (a) CO₂ monomolecular ions were irradiated onto an silicon substrate at a dose rate of 1×10¹⁶ ions/cm², and (b) CO₂ cluster ions having a number of component molecules (cluster size) of at least 500 were irradiated.

[0032] In FIG. 1(a), CO₂ monomolecular ions were irradiated under an acceleration voltage of 10 kV onto a silicon (001) substrate. An amorphous silicon layer of a thickness of 19 nm was formed on the substrate surface, and irregularities occurred on the interface between the amorphous layer and the substrate. The amorphous silicon layer is a damaged layer formed when CO₂ ions ejected onto the substrate surface hit atoms of the substrate. These irregularities of the interface and the damaged layer, causing deterioration of semiconductor device properties, must be converted to monocrystalline state. It is however very difficult to achieve complete conversion even through heat treatment at a temperature of at least 800° C.

[0033] In the case of FIG. 1(b), on the other hand, CO₂ cluster ions having a number of component atoms (cluster size) of at least 500 were accelerated under conditions including a dose rate of 1×10¹⁶ ions/cm2 and an acceleration voltage of 10 kV, and then, only clusters of a number of component molecules of at least 500 were irradiated onto a silicon (001) substrate at the room temperature by the retarding-field method. A silicon oxide film having a thickness of 10 nm was formed on the substrate surface, and no damaged layer was observed between the silicon oxide film and the silicon substrate, and the interface in between was very smooth.

[0034] The above-mentioned findings are confirmed also from FIG. 2. FIG. 2 is an infrared absorption spectrum of a thin film formed by irradiating CO₂ monomer ions and cluster ions under conditions including an acceleration voltage of 10 kV and a dose rate of 2×10¹⁶ ions/cm onto a silicon substrate surface. While a silicon oxide film only of the order of spontaneous oxide film can be formed by irradiation with CO₂ monomer ions, irradiation with cluster ions permits formation of an oxide film of about 70 A.

EXAMPLE 2

[0035]FIG. 3 illustrates changes in the concentration of Ni impurities as measured with various dose rates of CO₂ ions irradiated onto a silicon substrate, ranging from 0 to 2×10¹⁵ ions/cm². In this case, CO₂ cluster ions having a number of component molecules of at least 250 and CO₂ monomer ions were irradiated under an acceleration voltage of 10 kV onto the silicon substrates to which Ni had previously been forcedly deposited to give of 6×10¹² atoms/cm², and changes in the concentration of Ni impurities was measured before and after irradiation through measurement by the total reflection X-ray flourescence analysis method. While the concentration of Ni impurities does not depend upon the extent of dose rate in the irradiation of monomer ions, the concentration of Ni impurities decreases along with the increase in the dose rate in the irradiation of cluster ions.

[0036] As is clear from these results, in the case of irradiation of cluster ions, it is possible to reduce the concentration of impurities by increasing the dose rate of irradiated ions. The higher impurities-removing effect in the irradiation of cluster ions as compared to the irradiation of monomer (monomolecular) ions is attributable to the fact that, while embedding of impurities adhering to the surface into the interior of the substrate occurs in the irradiation of monomer ions, sputtering from the irradiation onto the substrate preferentially eliminates impurities on the substrate surface in the irradiation of cluster ions.

[0037] As described above, the method of the present invention, permitting formation of a clean surface free from defects on the substrate surface, makes it possible to manufacture a high-quality thin film with the slightest content of impurities, useful for a semiconductor circuit device.

EXAMPLE 3

[0038] Table 1 shows, for an SiO₂ film formed on a polycrystalline silicon film irradiated with CO₂ cluster ions under conditions including an acceleration voltage of 10 kV and a dose rate of 5×10¹⁵ ions/cm², the film thickness and the average surface roughness before and after a treatment with fluoric acid solution. The treatment with fluoric acid solution was applied until complete elimination of the SiO₂ film. For comparison purposes, values obtained with a polycrystalline silicon film not irradiated with cluster ions are also shown. TABLE 1 Average surface roughness (unit: Å) Before After Thickness of fluoric fluoric SiO₂ acid acid Kind of sample thin film (nm) treatment treatment Sample not irradiated — 37 37 Sample irradiated with 8 7 9 cluster ions of size of 250 Sample irradiated with 6 18 20 cluster ions of size of 500

[0039] The sample not irradiated with cluster ions has an average surface roughness of 37 A, and this value does not vary with the fluoric acid treatment. Irradiation of cluster ions of a size of 250 and 500 reduces the average surface roughness of the polycrystalline silicon film to 7 Å and 18 Å, respectively. At the same time, an SiO₂ thin film having a thickness of from 8 to 6 nm is formed on the surface of the polycrystalline silicon film, thus forming an SiO₂ thin film simultaneously with planarization. A satisfactory surface planarity is kept with almost no change by the fluoric acid treatment. Thus, the irradiation of CO₂ cluster ions permits formation of a silicon oxide film which has a planar surface, with a smooth interface between said film and the silicon substrate, and has a uniform thickness.

EXAMPLE 4

[0040] Conditions for generating cluster of various gases were evaluated. As shown in FIG. 4, cooled dry nitrogen gas was passed through a piping (2) attached to a nozzle (1) section which was then cooled. This permitted ionization of clusters of gases which had not been possible at room temperature.

[0041]FIG. 5 is a graph illustrating the relationship between the intensity of oxygen (O₂) gas cluster beam and the supply pressure at various nozzle temperatures. FIG. 6 is a graph illustrating such relationship for nitrogen (N₂) gas. FIG. 7 illustrates the relationship for Ar gas for reference.

[0042] These results suggest the possibility of determining the degree of easiness of generation of the cluster of a gas applicable in the present invention by means of the following formula:

[0043] Easiness of Generation of Cluster

ψ=P ₀ ·d ₀·(T _(b) /T _(N))^(γ/(γ−)1)

[0044] where,

[0045] P₀: gas supply pressure,

[0046] d₀: inside diameter of nozzle throat,

[0047] T_(b): boiling point of gas,

[0048] T_(N): ejecting temperature of gas,

[0049] γ: ratio of specific heats of gas (specific heat at constant pressure/isometric specific heat).

[0050] Larger P₀ or T_(b) leads to easier generation of clusters.

[0051] Smaller T_(N) or γ corresponds to easier generation of clusters.

[0052] In all the cases described above, various gases were effectively utilized for the formation of thin films.

EXAMPLE 5

[0053]FIG. 8 is an infrared absorption spectrum of a thin film formed by irradiating O₂ cluster ions having a number of component molecules (cluster size) of at least 250 onto an Si substrate surface at room temperature under an acceleration voltage of 4 kV and at a dose rate of 1×10¹⁵ ions/cm². It is clear from FIG. 8 that an oxide film of about 40 A was formed with a lower acceleration voltage and a lower dose rate than the conditions for irradiation of CO₂ cluster ions.

EXAMPLE 6

[0054] Table 2 shows the results of investigation by the photoelectron spectroscopy method of an oxide film formed by irradiating CO₂ cluster ions and O₂ cluster ions under the same irradiating conditions including an acceleration voltage of 4 kV, a cluster size of at least 250, and a dose rate of 1×10¹⁵ ions/cm². The results of investigation on a clean Si substrate not irradiated are also shown for reference. By using O₂ cluster ions, it is possible to form a thicker oxide film as compared with the case of CO₂ cluster ions. It is also clear that the relative intensity of emission spectrum from Cls core level of the oxide film formed by the irradiation of O₂ clusters is of the same order as that for a substrate not irradiated and the carbon content is almost nil, even with the largest film thickness. When presence of residual carbon in the film is not desired, it is suggested to be effective to use O₂ cluster ions to obtain a thicker oxide film. TABLE 2 Raw material gas Oxide film of irradiated thickness Relative intensity cluster ions (unit: A) of C1s signal O₂ 40 31 CO₂ 18 44 Substrate not 8 29 irradiated

[0055] According to the present invention, as described above in detail, it is possible to form a high-quality reactive ultra-thin film, with no damage to the substrate, having a very smooth interface, even at the room temperature, by irradiating cluster ions of a reactive substance onto the substrate surface to cause reaction. 

What is claimed is:
 1. A method for forming a thin film on the surface of a substrate with a gas cluster ion beam, which comprises the step of irradiating the surface of a substrate with ions of a gas cluster (which is a massive group of atoms or molecules of a reactive substance taking the gseous form at room temperature under atmospheric pressure) to cause a reaction with the surface of a substrate and thereby form a thin film on the substrate surface.
 2. A method as claimed in claim 1, wherein the said reactive substance is oxygen, an oxide, nitrogen, a nitride, a carbide, a sulfide, a halide, a hydride, an organo metallic compound, a mixture thereof or a mixture thereof with an inert gas.
 3. A method as claimed in claim 1, wherein the gas cluster ions are selected in terms of the number of component molecules.
 4. A method as claimed in 1, wherein impurities present on the substrate are removed by the irradiation with the gas cluster ion beam, thereby forming a thin film with a low content of impurities.
 5. A method as claimed in claim 1, wherein the substrate surface is planarized (made flat) by the irradiation with the gas cluster ions, at the same time as the formation of the thisn film.
 6. A method as claimed in claim 1, wherein the thin film formed is an oxide.
 7. A method as claimed in claim 1, wherein the substrate is silicon.
 8. A method as claimed in claim 1, wherein the reactive substance is oxygen, an oxide of carbon, an oxide of nitrogen, a mixture thereof, or a mixture thereof with a rare (inert) gas, and the thin film formed is an oxide film.
 9. A method as claimed in claim 1, wherein the gas cluster is generated by supplying a pressurized gas to an expansion type nozzle cooled by a coolant.
 10. A method as claimed in claim 1, which includes the steps of (i) forming a gas cluster from a substance which is reactive with the surface of the substrate and is a gas at room temperature and atmospheric pressure, (ii) ionizing the gas cluster, and (iii) irradiating the surface of the substrate with a beam of gas cluster ions.
 11. A method for generating gas cluster ions, which comprises the steps of generating a gas cluster with an expansion type nozzle cooled by a coolant and ionizing the resultant cluster. 